Electroplating composition and method for depositing a chromium coating on a substrate

ABSTRACT

The present invention refers to an electroplating composition for depositing a chromium coating on a substrate, the composition including:(i) trivalent chromium ions,(ii) at least one complexing agent for the trivalent chromium ions, and(iii) at least one additive selected from the group consisting of betaines, polymeric glycols, monomeric diols, and mixtures thereof.

FIELD OF THE INVENTION

The present invention relates to an electroplating composition andmethod for depositing a chromium coating on a substrate. In particularthe electroplating composition according to the present invention allowsfor the electrolytic deposition of functional chromium coatings, alsocalled hard chromium coatings, on a substrate, in particular on aferrous substrate, in particular on a nickel or nickel alloy coatedferrous substrate.

BACKGROUND OF THE INVENTION

Functional chromium coatings usually have a much higher average coatingthickness, typically from at least 1 μm up to several hundreds of micrometers, compared to decorative chromium coatings, typically below 1 μm,and are characterized by excellent hardness and wear resistance.

Functional chromium coatings obtained from a chromium electroplatingcomposition containing hexavalent chromium are known in the prior artand are a well-established standard.

During recent decades, hexavalent chromium-based methods are more andmore replaced by trivalent chromium-based methods, which are much morehealth-and environment friendly.

Typical chromium-based electroplating methods are described in thefollowing prior art.

WO 2015/110627 A1 refers to an electroplating composition for depositingchromium and to a method for depositing chromium on a substrate usingsaid electroplating composition.

U.S. Pat. No. 2,748,069 relates to an electroplating solution ofchromium, which allows obtaining very quickly a chromium coating of verygood physical and mechanical properties. The chromium plating solutioncan be used for special electrolyzing methods, such as those known asspot or plugging or penciling galvanoplasty. In such special methods thesubstrate is typically not immersed into a respective electroplatingsolution.

WO 2018/185154 A1 discloses a method for electrolytically depositing achromium or chromium alloy coating on a substrate.

U.S. Pat. No. 4,009,085 discloses lubricating compositions and a processfor treating a metal sheet to impart lubricity and abrasion resistancethereto.

U.S. Pat. No. 3,432,408 A relates to the prevention of mist and spray inacidic hexavalent electroplating baths.

US 2016/068983 A1 refers to methods and plating baths forelectrodepositing a dark chromium layer on a workpiece, the bath forexample comprising a diol.

EP 0 100 133 A1 refers to zinc and nickel tolerant trivalent chromiumplating baths and plating process, utilizing for example a betaine.

CN 108034969 A refers to a sulfate-based trivalent chromiumelectroplating bath comprising for example a diol or polyethyleneglycol.

US 2018/245227 A1 relates to the use of ionic liquids in electroplating,and in particular for electroplating thick, hard chromium from trivalentsalts.

However, when using trivalent chromium-based methods for electroplatingaccording to the prior art, the cathodic current efficiencies (CCE)observed are typically smaller compared to the cathodic currentefficiencies observed when using hexavalent chromium-based methods forelectroplating.

The cathodic current efficiency (CCE) is based on Faraday's law and isdescribed as the percentage of metal actually deposited on the substrateduring electroplating compared to the theoretical ideal case, when all,i.e. 100%, of the metal present in the electroplating composition couldbe deposited on the substrate. In hexavalent chromium-based methods forelectroplating according to the prior art typical CCEs are between 20%and 25%, while in trivalent chromium-based methods for electroplatingaccording to the prior art typical CCEs can be as low as 10%.

An important factor, which can influence the CCEs in trivalentchromium-based methods for electroplating are inter alia the types andconcentrations of complexing agents, which are used to stabilize thetrivalent chromium ions in the electroplating composition. Otherfactors, which can influence the CCEs in trivalent chromium-basedmethods for electroplating are inter alia the types and concentrationsof further additives, which can be added to the electroplatingcomposition.

OBJECTIVE OF THE PRESENT INVENTION

It was therefore the first objective of the present invention to providean electroplating composition comprising trivalent chromium ions and arespective method for depositing a chromium coating on a substrate withimproved qualities of the coating (e.g. such as hardness and/or wearresistance).

It was therefore the second objective of the present invention toprovide an electroplating composition comprising trivalent chromium ionsand a respective method for depositing a chromium coating on a substrateresulting in an increased cathodic current efficiency (CCE).

SUMMARY OF THE INVENTION

The objectives mentioned above are solved according to a first aspect byan electroplating composition for depositing a chromium coating on asubstrate, the composition comprising:

-   (i) trivalent chromium ions,-   (ii) at least one complexing agent for the trivalent chromium ions,    and-   (iii) at least one additive selected from the group consisting of    betaines, polymeric glycols, monomeric diols, and mixtures thereof.

By utilizing the at least one complexing agent for the trivalentchromium ions, a particularly effective stabilization of trivalentchromium ions in the electroplating composition can be achieved, whichallows for an effective deposition of the chromium coating on thesubstrate during electroplating.

It has been surprisingly found that using at least one additive selectedfrom the group consisting of betaines, polymeric glycols, monomericdiols, and mixtures thereof, a significant increase of the cathodiccurrent efficiency (CCE) during electroplating is observed, while on theother hand a coating with high quality is obtained.

The objectives mentioned above are solved according to a second aspectby a method for depositing a chromium coating on a substrate, the methodcomprising the following steps:

-   (a) providing the substrate,-   (b) providing an electroplating composition for depositing a    chromium coating on the substrate, the composition comprising:    -   (i) trivalent chromium ions,    -   (ii) at least one complexing agent for the trivalent chromium        ions, and    -   (iii) at least one additive selected from the group consisting        of betaines, polymeric glycols, monomeric diols, and mixtures        thereof.-   (c) contacting the substrate with said electroplating composition    and applying an electrical current such that the chromium coating is    deposited on at least one surface of said substrate.

By having the at least one additive as defined above in theelectroplating composition, a comparatively high cathodic currentefficiency (CCE) is obtained during step (c).

The objectives mentioned above are solved according to a third aspect bya substrate with a surface, wherein the surface of the substratecomprises a chromium coating obtained by a method for depositingaccording to the second aspect.

The objectives mentioned above are solved according to a fourth aspectby a use of at least one additive selected from the group consisting ofbetaines, polymeric glycols, monomeric diols, and mixtures thereof, forincreasing the cathodic current efficiency in an electroplatingcomposition for depositing a chromium coating on a substrate.

BRIEF DESCRIPTION OF THE TABLE

In Table 1, correlations between varying concentrations of differentadditives in respective electroplating compositions, represented as thetotal amount of additive in g/L, based on the total volume of therespective electroplating composition, in respect to the resultingcathodic current efficiency (CCE) are shown. Each bar (C1) to (18)represents a specific electroplating composition. Further details aregiven in the “Examples” section below in the text.

DETAILED DESCRIPTION OF THE INVENTION

In the context of the present invention, the term “at least one” or “oneor more” denotes (and is exchangeable with) “one, two, three or more”and “one, two, three or more than three”, respectively. Furthermore,“trivalent chromium” refers to chromium with the oxidation number +3.The term “trivalent chromium ions” refers to Cr³⁺-ions in a free orcomplexed form. Furthermore, “hexavalent chromium” refers to chromiumwith the oxidation number +6.

The term C_(X)-C_(y) if used in the context of the present inventionrefers to a compound comprising a total number from “x” carbon atoms to“y” carbon atoms. For example, the term C₁-C₂₅ diols refers to diolscomprising a total number from 1 carbon atom to 25 carbon atoms.

The cathodic current efficiency (CCE) is determined as described in thetext above and is based on gravimetric analysis (see examples below inthe text).

The electroplating composition of the present invention comprises atleast one additive selected from the group consisting of betaines,polymeric glycols, monomeric diols and mixtures thereof.

In rare cases an electroplating composition of the present invention ispreferred with the proviso that the at least one additive does notcomprise polymeric glycols. Preferably, the electroplating compositionis substantially free of or does not comprise polyethylene glycol, morepreferably is substantially free of or does not comprise polyalkyleneglycols, most preferably is substantially free of or does not comprisepolymeric glycols. In such a case, the electroplating composition of thepresent invention preferably comprises at least one additive selectedfrom the group consisting of betaines, monomeric diols, and mixturesthereof.

In very rare cases an electroplating composition of the presentinvention is preferred with the proviso that the at least one additivedoes not comprise monomeric diols. Preferably, the electroplatingcomposition is substantially free of or does not comprise ethanediol,more preferably is substantially free of or does not comprisealkanediols, even more preferably is substantially free of or does notcomprise monomeric diols. In such a case, the electroplating compositionof the present invention preferably comprises at least one additiveselected from the group consisting of betaines, polymeric glycols, andmixtures thereof

In the electroplating composition of the present invention no hexavalentchromium is intentionally added to the electroplating composition. Thus,the electroplating composition is substantially free of or does notcomprise hexavalent chromium (except very small amounts which may beformed anodically).

Preferably, the electroplating composition of the present invention isan aqueous electroplating composition comprising trivalent chromiumions. Preferably, the electroplating composition comprises furtheradditives and/or metal ions, more preferably iron ions, nickel ions,copper ions and/or zinc ions.

In the context of the present invention, the chromium coating compriseschromium alloys, i.e. a coating comprising not only chromium but alsoalloying elements. In very rare cases, metal alloying elements arepreferred, preferably from metal ions as mentioned above. More typicaland preferred are non-metal alloying elements, preferably carbon,nitrogen, and/or oxygen.

The electroplating composition of the present invention is preferablyused more than one time for depositing a chromium coating on a pluralityof different substrates, preferably during a continuous process.Preferably, the electroplating composition is repeatedly utilized duringelectroplating, preferably for a usage of at least 100 Ah per literelectroplating composition, preferably at least 150 Ah per liter, morepreferably at least 200 Ah per liter, most preferably at least 300 Ahper liter.

Basically, preferred is an electroplating composition of the presentinvention, wherein the betaines comprise at least 5 carbon atoms, morepreferably at least 10 carbon atoms, and even more preferably at least15 carbon atoms. Preferable not having more than 50 carbon atoms.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises at least one or morethan one betaine, preferably independently comprising at least 5 carbonatoms, more preferably at least 10 carbon atoms, and even morepreferably at least 15 carbon atoms. Preferable not having more than 50carbon atoms.

More preferred is an electroplating composition of the presentinvention, wherein the electroplating composition comprises at least oneor more than one betaine independently comprising at least 10 carbonatoms, preferably at least 15 carbon atoms. Preferable not having morethan 50 carbon atoms.

This all means that out of the selection from betaines, polymericglycols, monomeric diols, and mixtures thereof, the electroplatingcomposition of the present invention must at least comprise a betaine,preferably with the number of carbon atoms as defined above.

Preferred is an electroplating composition of the present invention,wherein the betaines independently comprise (or betaines at leastdenote):

-   -   a positively charged quaternary nitrogen atom, and    -   a negatively charged sulfonate group and/or negatively charged        carboxylate group,        with the proviso that the positive charge cannot be removed by        deprotonation.

Preferred is an electroplating composition of the present invention,wherein the betaines have a neutral net charge.

Preferred is an electroplating composition of the present invention,wherein the betaines are linear betaines.

Preferred is an electroplating composition of the present invention,wherein the betaines do not comprise an aromatic ring structure,preferably do not comprise a ring structure.

Preferred is an electroplating composition of the present invention,wherein the positively charged quaternary nitrogen atom has substituentssuch that said positive charge results, with the proviso that thesubstituents are not hydrogen.

Preferably, said substituents are independently selected from the groupconsisting of alkyl, ester, and amide.

Preferably, alkyl comprises C₁-C₂₀ alkyl, more preferably C₁-C₁₇ alkyl,most preferably C₁-C₁₅ alkyl.

Preferably, ester comprises C₈-C₂₀ ester, more preferably C₉-C₁₇ ester,most preferably C₁₀-C₁₆ ester.

Preferably, ester comprises fatty acid esters, most preferably with anumber of carbon atoms as defined above.

Preferably, amide comprises C₈-C₂₀ amide, more preferably C₉-C₁₇ amide,most preferably C₁₀-C₁₆ amide.

Preferably, amide comprises fatty acid amides, most preferably with anumber of carbon atoms as defined above for amide.

Most preferably, at least one, preferably two, substituent is alkyl,preferably as defined above, most preferably a C₁-C₅ alkyl, even moremost preferably a C₁-C₃ alkyl, and in addition at least one substituentis ester, preferably as defined above, or amide, preferably as definedabove.

Preferred is an electroplating composition of the present invention,wherein the betaines independently comprise a positively chargedquaternary nitrogen atom and a negatively charged sulfonate group, withthe proviso that the positive charge cannot be removed by deprotonation.

Preferred is an electroplating composition of the present invention,wherein the betaines are selected from the group consisting ofN-substituted-ammonium sulfobetaines and N-substituted-ammoniumcarboxybetaines.

Preferred is an electroplating composition of the present invention,wherein the betaines are selected from the group consisting ofN-substituted-N,N-Dialkyl -ammonium sulfobetaines, preferablyN-substituted-N, N-Dialkyl-N-alkyl ammonium sulfobetaines.

Preferred is an electroplating composition of the present invention,wherein the N-substituted-N,N-Dialkyl-ammonium sulfobetaines and theN-substituted-N,N -Dialkyl-N-alkyl ammonium sulfobetaines, respectively,is N-substituted with a substituent selected from the group consistingof alkyl, and amidoalkyl, wherein amidoalkyl is preferablycocoamidopropyl.

Preferred is an electroplating composition of the present invention,wherein the betaines comprise one or more ofN,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-hydroxy-3-sulfopropyl) ammoniumbetaine, N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Decyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Octadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, andN,N-Dimethyl-N-dodecylglycine betaine.

By selecting the preferred betaines a particular advantageous cathodiccurrent efficiency (CCE) is achieved, preferably along with a foamformation for mist suppression during electroplating. Advantageously,the concentration of betaines can be comparatively low (see examples).

Regarding the at least one additive, in some cases an electroplatingcomposition of the present invention is preferred, wherein theelectroplating composition comprises at least one betaine and preferablyin addition one or more than one of polymeric glycols and/or monomericdiols.

In particular preferred is an electroplating composition of the presentinvention, wherein the electroplating composition comprises at least onebetaine and in addition one or more than one monomeric diol.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises the betaines in a totalconcentration in a range from 0.0005 g/L to 1 g/L, based on the totalvolume of the electroplating composition, preferably from 0.001 g/L to0.5 g/L, more preferably from 0.005 g/L to 0.3 g/L, and most preferablyfrom 0.01 g/L to 0.2 g/L.

Preferred is an electroplating composition of the present invention,wherein the polymeric glycols are polyalkylene glycols, preferablypolyethylene glycols. Preferably, the polyalkylene glycols, preferablythe polyethylene glycols, have an average molecular weight in a rangefrom 150 Da to 5000 Da, preferably from 200 Da to 2500 Da. Mostpreferred are polymeric glycols, selected from the group consisting ofpolyethylene glycol 200, polyethylene glycol 600, and polyethyleneglycol 1500.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises the polymeric glycolsin a total concentration in a range from 0.01 g/L to 50 g/L, based onthe total volume of the electroplating composition, preferably from 0.05g/L to 35 g/L, more preferably from 0.1 g/L to 20 g/L, most preferablyfrom 0.15 g/L to 25 g/L.

Also, when using polymeric glycols as a representative of the at leastone additives a beneficial increase in the cathodic current efficiency(CCE) is obtained.

Preferred is an electroplating composition of the present invention,wherein the monomeric diols are C₁-C₂₅ diols, preferably C₂-C₂₃ diols,more preferably C₂-C₂₁ diols, even more preferably C₂-C₁₉ diols, mostpreferably C₂-C₁₈ diols.

In some cases, preferred is an electroplating composition of the presentinvention, wherein the monomeric diols comprise one or more than oneC₁-C₁₀ diol, preferably one or more than one C₂-C₈ diol, more preferablyone or more than one C₂-C₆ diol, even more preferably one or more thanone C₂-C₅ diol, most preferably one or more than one C₂-C₄ diol, evenmost preferably the monomeric diol comprises 1,2-propane diol and/or1,3-propane diol. Most preferably in combination with one or more thanone betaine.

In some cases, preferred is an electroplating composition of the presentinvention, wherein the monomeric diols comprise, in addition to theC₁-C₁₀ diols and its preferred variants, or alternatively to the C₁-C₁₀diols, and its preferred variants, preferably in addition, one or morethan one C₁₁-C₂₅ diol, preferably one or more than one C₁₂-C₂₃ diol,more preferably one or more than one C₁₃-C₂₁ diol, even more preferablyone or more than one C₁₄-C₂₀ diol, most preferably one or more than oneC₁₅-C₁₉ diol, even most preferably one or more than one C₁₆-C₁₈ diol,most preferably in combination with one or more than one betaine.

Preferred is an electroplating composition of the present invention,wherein the one or more than one C₁₁-C₂₅ diol, and its preferredvariants, is selected from the group of saturated and unsaturatedC₁₁-C₂₅ diols, preferably saturated C₁₁-C₂₅ diols.

Preferred is an electroplating composition of the present invention,wherein the one or more than one C₁₁-C₂₅ diol, and its preferredvariants, is branched.

Preferred is an electroplating composition of the present invention,wherein the one or more than one C₁₁-C₂₅ diol, and its preferredvariants, comprises one, two or more than two iso-propyl moieties.

Preferred is an electroplating composition of the present invention,wherein the one or more than one C₁₁-C₂₅ diol, and its preferredvariants, comprises an anti-foam compound. In many cases, such C₁₁-C₂₅diols have the potential to function as an anti-foam compound.Furthermore, in many cases C₁ to C₁₀ diols are often excellent solventsfor aforementioned anti-foam compounds.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises the monomeric diols ina total concentration in a range from 0.001 g/L to 60 g/L, based on thetotal volume of the electroplating composition, preferably from 0.1 g/Lto 50 g/L, more preferably from 1.0 g/L to 40 g/L, even more preferablyfrom 5.0 g/L to 35 g/L, most preferably from 15 g/L to 30 g/L.

In a few specific cases, preferred is an electroplating composition ofthe present invention, wherein the electroplating composition comprisesthe monomeric diols in a total concentration in a range from 0.001 g/Lto 10 g/L, based on the total volume of the electroplating composition,preferably from 0.01 g/L to 8.0 g/L, more preferably from 0.1 g/L to 6.0g/L, even more preferably from 0.5 g/L to 4.0 g/L, most preferably from1.0 g/L to 3.0 g/L. This most preferable selection preferably applies ifthe electroplating composition of the present invention comprises one ormore than one betaine, most preferably if the electroplating compositionof the present invention comprises one or more than one betaine and oneor more than one C₁₁-C₂₅ diol, and its preferred variants.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition is substantially free of or doesnot comprise hexavalent chromium ions.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises trivalent chromium ionsin a total concentration from 10 g/L to 30 g/L, based on the totalvolume of the electroplating composition, preferably from 14 g/L to 27g/L, and most preferably from 17 g/L to 24 g/L.

With the preferred selections of the concentration range of thetrivalent chromium ions in the electroplating composition, a particulareffective deposition of the chromium coating on the substrate can beachieved. If the total amount of trivalent chromium ions issignificantly below 10 g/L in many cases an insufficient deposition isobserved, and the deposited chromium is usually of low quality. If thetotal amount is significantly above 30 g/L, the electroplatingcomposition is not any longer stable, which includes formation ofundesired precipitates.

Preferred is an electroplating composition of the present invention,wherein the at least one complexing agent for the trivalent chromiumions is selected from the group consisting of organic complexing agentsand salts thereof, preferably carboxylic acids and salts thereof, morepreferably aliphatic carboxylic acids and salts thereof, most preferablyaliphatic mono-carboxylic acids and salts thereof. Preferred aliphaticmono-carboxylic acids and salts thereof are C₁-C₁₀ aliphaticmono-carboxylic acids and salts thereof, preferably C₁-C₈ aliphaticmono-carboxylic acids and salts thereof, more preferably C₁-C₆ aliphaticmono-carboxylic acids and salts thereof, most preferably C₁-C₃ aliphaticmono-carboxylic acids and salts thereof.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition comprises the at least onecomplexing agent in a total concentration in a range from 50 g/L to 350g/L, based on the total volume of the composition, preferably from 100g/L to 300 g/L, even more preferably from 150 g/L to 250 g/L.

By utilizing in particular the above-mentioned preferred selection ofcomplexing agents, the trivalent chromium ions can be efficientlystabilized in the electroplating composition by the complexing agents

Preferred is an electroplating composition of the present invention,wherein the electroplating composition has a pH in a range from 4.1 to7.0, preferably from 4.5 to 6.5, more preferably from 5.0 to 6.0, andmost preferably from 5.3 to 5.9.

The preferred acidic pH ranges are in particular beneficial foreffectively depositing a chromium coating on the substrate having thedesired qualities.

In particular preferred is an electroplating composition of the presentinvention, wherein the electroplating composition comprises one or morethan one betaine and one or more than one monomeric diol, with theproviso that the one or more than one monomeric diol comprises one ormore than one C₁₁-C₂₅ diol comprising one, two or more than twoiso-propyl moieties.

Even more preferred is an electroplating composition of the presentinvention, wherein the electroplating composition comprises one or morethan one betaine and more than one monomeric diol, with the proviso that

-   -   at least one of the more than one monomeric diol comprises one        or more than one C₁₁-C₂₅ diol (or another preferred diol among        the C₁₁-C₂₅ diol as mentioned above) comprising one, two or more        than two iso-propyl moieties, and    -   at least one of the more than one monomeric diol comprises one        or more than one C₂-C₈ diol (or another preferred diol among the        C₂-C₈ diol as mentioned above).

Preferred is an electroplating composition of the present invention,wherein the electroplating composition is essentially free of or doesnot comprise boric acid, preferably is essentially free of or does notcomprise boron-containing compounds.

Boron-containing compounds are not desired because they areenvironmentally problematic. When using boron-containing compounds,waste water treatment is expensive and time consuming. Furthermore,boric acid typically shows poor solubility and therefore has thetendency to form precipitates. Although such precipitates can besolubilized upon heating, a respective electroplating composition cannotbe utilized for electroplating during this time. There is a significantrisk that such precipitates facilitate a reduced chromium coatingquality. Thus, the electroplating composition of the present inventionis preferably essentially free of or does not comprise boron-containingcompounds. Surprisingly, the electroplating composition of the presentinvention performs very well without boron-containing compounds, inparticular in the above-mentioned preferred pH ranges.

Preferred is an electroplating composition of the present invention,wherein the electroplating composition is essentially free of or doesnot comprise organic compounds containing divalent sulfur, preferably isessentially free of or does not comprise sulfur-containing compoundswith a sulfur atom having an oxidation number below +6.

Omitting organic compounds containing divalent sulfur from theelectroplating composition is particularly advantageous when employingthe electroplating composition for deposition of hard, functionalchromium coatings.

The term “does not comprise” typically denotes that respective compoundsand/or ingredients are not intentionally added to e.g. theelectroplating composition. This does not exclude that such compoundsare dragged in as impurities of other chemicals. However, typically thetotal amount of such compounds and ingredients is below the detectionrange and therefore is not critical in the various aspects of thepresent invention.

Preferred is an electroplating composition furthermore comprising one ormore than one compound selected from the group consisting of

-   -   one or more than one type of halogen ions, preferably bromide,    -   one or more than one type of alkaline metal cations, preferably        sodium and/or potassium,    -   sulfate ions, and    -   ammonium ions.

By adding one or more of the above-mentioned compounds the deposition ofchromium during an electroplating process can be improved, mostpreferably during the method of the present invention.

Preferably, the electroplating composition of the present inventioncomprises one or more than one type of halogen ions, preferably bromide,in a concentration of at least 0.06 mol/L, based on the total volume ofthe electroplating composition, more preferably at least 0.1 mol/L, evenmore preferably at least 0.15 mol/L. In particular bromide anionseffectively suppress the formation of hexavalent chromium species at theat least one anode.

Preferably, the electroplating composition comprises one or more thanone type of alkaline metal cations, preferably sodium and/or potassium,in a total concentration ranging from 0 mol/L to 0.5 mol/L, based on thetotal volume of the electroplating composition, more preferably from 0mol/L to 0.3 mol/L, even more preferably from 0 mol/L to 0.1 mol/L, andmost preferably from 0 mol/L to 0.08 mol/L.

Typically, rubidium, francium, and caesium ions are not utilized in anelectroplating composition comprising trivalent chromium ions. Thus,preferably the one or more than one type of alkaline metal cationsincludes metal cations of lithium, sodium, and potassium, mostly sodiumand potassium.

Preferred is an electroplating composition of the present invention,wherein the trivalent chromium ions of the electroplating compositionare obtained from a soluble, trivalent chromium ion containing source,typically a water-soluble salt comprising said trivalent chromium ions.Preferably, the soluble, trivalent chromium ion containing sourcecomprises alkali metal cations in a total amount of 1 wt.-% or less,based on the total weight of said source. In some cases, preferably,such a source is utilized for replenishing trivalent chromium ions ifthe method is operated continuously. A preferred water-soluble saltcomprising said trivalent chromium ions is alkali metal free trivalentchromium sulfate or alkali metal free trivalent chromium chloride. Insome cases it is preferred that the electroplating composition of thepresent invention contains sulfate ions, preferably in a total amount inthe range from 50 g/L to 250 g/L, based on the total volume of theelectroplating composition.

Preferably, the soluble, trivalent chromium ion containing sourcecomprises or is chromium sulfate, more preferably acidic chromiumsulfate, even more preferably chromium sulfate with the general formulaCr₂(SO₄)₃ and a molecular weight of 392 g/mol.

More preferably, for replenishing, a soluble, trivalent chromium ioncontaining source is preferred, wherein the anion is an organic anion,preferably an organic acid anion, most preferably formate and/oracetate.

The present invention according to the second aspect provides a methodfor depositing a chromium coating on a substrate, the method comprisingthe following steps:

-   (a) providing the substrate,-   (b) providing an electroplating composition (preferably as described    above, most preferably as above described as being preferred) for    depositing a chromium coating on the substrate, the composition    comprising:    -   (i) trivalent chromium ions,    -   (ii) at least one complexing agent for the trivalent chromium        ions, and    -   (iii) at least one additive selected from the group consisting        of betaines, polymeric glycols, monomeric diols, and mixtures        thereof.-   (c) contacting the substrate with said electroplating composition    and applying an electrical current such that the chromium coating is    deposited on at least one surface of said substrate.

Preferably, the aforementioned regarding the electroplating compositionof the present invention (preferably as described above as beingpreferred), applies likewise to the method of the present invention(preferably a method as described below as being preferred).

Preferred is a method of the present invention, wherein in step (c) theelectrical current is a direct current.

Preferably, the direct current (DC) is a direct current withoutinterruptions during the electroplating, wherein more preferably thedirect current is not pulsed (non-pulsed DC). Furthermore, the directcurrent preferably does not include reverse pulses.

Preferred is a method of the present invention, wherein in step (c) theelectrical current has a cathodic current density of at least 18 A/dm²,preferably of at least 20 A/dm², more preferably of at least 25 A/dm²,even more preferably of at least 30 A/dm², most preferably of at least39 A/dm². Preferably, the cathodic current density is in a range from 18A/dm² to 60 A/dm², more preferably from 25 A/dm² to 55 A/dm², mostpreferably from 30 A/dm² to 50 A/dm².

Typically, the substrate provided during the method of the presentinvention is the cathode during the electroplating process (i.e. in step(c)). Preferably, more than one substrate is provided simultaneously instep (c) of the method of the present invention.

Preferred is a method of the present invention, wherein in step (c) atleast one anode is provided, wherein the at least one anode isindependently selected from the group consisting of graphite anodes andmixed metal oxide on titanium anodes. Such anodes have shown to besufficiently resistant in the electroplating composition of the presentinvention. Preferably, the at least one anode does not comprise any leador chromium.

In step (c) of the method of the present invention a chromium coating isdeposited, either a pure one or an alloy. Preferably, the chromiumcoating is an alloy. Preferred alloying elements are carbon, nitrogen,and oxygen, preferably carbon and oxygen. Carbon is typically presentbecause of organic compounds usually present in the electroplatingcomposition. In many cases preferred is a method of the presentinvention, wherein the chromium coating does not comprise one, more thanone or all elements selected from the group consisting of sulphur,nickel, copper, aluminium, tin and iron. More preferably, the onlyalloying elements are carbon, nitrogen, and/or oxygen, more preferablycarbon and/or oxygen, most preferably carbon and oxygen. Preferably, thechromium coating contains 90 weight percent chromium or more, based onthe total weight of the chromium coating, more preferably 95 weightpercent or more.

Preferred is a method of the present invention, wherein in step (c) theelectroplating composition has a temperature in a range from 20° C. to90° C., preferably from 30° C. to 70° C., more preferably from 40° C. to60° C., most preferably from 45° C. to 58° C.

In the preferred temperature range (in particular in the most preferredtemperature range) the chromium coating is optimally deposited in step(c). If the temperature significantly exceeds 90° C., an undesiredvaporization occurs, which can negatively affect the concentration ofthe composition components. Furthermore, the undesired anodic formationof hexavalent chromium is significantly less suppressed. If thetemperature is significantly below 20° C. the electrodeposition isinsufficient.

Preferred is a method of the present invention, wherein step (c) isperformed for a time period from 10 min to 100 min, preferably from 20min to 90 min, more preferably from 30 min to 60 min.

Preferred is a method of the present invention, wherein in step (c) theelectroplating composition is stirred, preferably with a stirring ratein a range from 100 rpm to 500 rpm, most preferably from 200 rpm to 400rpm.

By performing the method step (c) in the abovementioned preferredtemperature ranges and/or (preferably and) for the preferred timeperiods and/or (preferably and) with the preferred stirring rates,particularly advantageous electrodeposition kinetics during step (c) canbe ensured.

Preferred is a method of the present invention further comprising afterstep (c) step

-   -   (d) heat-treating the chromium-coated substrate obtained from        step (c).

Preferred is a method of the present invention, wherein in step (d) theheat-treating is carried out at a temperature in a range from 100° C. to250° C., preferably from 120° C. to 240° C., more preferably from 150°C. to 220° C., most preferably from 170° C. to 200° C.

Preferred is a method of the present invention, wherein in step (d) theheat-treating is carried out for a time period from 1 h to 10 h,preferably from 2 h to 4 h.

By preferably performing a heat-treatment of the substrate, morepreferably at the preferred temperatures and/or for the preferred timeperiods, the properties of the chromium coating can be further improvedin some cases (e.g. hardness).

Preferred is a method of the present invention, wherein in step (c) thecathodic current efficiency (CCE) is 11% or more, preferably 12% ormore, most preferably 13% or more. This most preferably applies, if anidentical method with the only exception that the electroplatingcomposition does not comprise an additive, is carried out.

By significantly increasing the cathodic current efficiency (CCE) to 11%or more, the entire method is more effective and economical.Furthermore, less energy is wasted and less hydrogen gas is producedduring step (c).

Preferred is a method of the present invention, wherein the substratecomprises a metal rod.

Preferred is a method of the present invention, wherein the substratecomprises a metal or metal alloy, preferably comprises one or more thanone metal selected from the group consisting of copper, iron, nickel andaluminum, more preferably comprises one or more than one metal selectedfrom the group consisting of copper, iron, and nickel, most preferablycomprises at least iron.

In many cases preferred is a substrate comprising a pre-coating, thepre-coating preferably being a nickel or nickel alloy coating, mostpreferably a semi-bright nickel coating, on which the chromium coatingis applied to during step (c) of the method of the present invention. Inparticular preferred is a steel substrate pre-coated with a nickel ornickel alloy coating. However, preferably other pre-coatings arealternatively or additionally present. In many cases such a pre-coatingsignificantly increases corrosion resistance compared to a metalsubstrate without such a pre-coating. However, in some cases thesubstrates are not susceptible to corrosion due to a corrosion inertenvironment (e.g. in an oil composition). In such a case a pre-coating,preferably a nickel or nickel alloy pre-coating, is not necessarilyneeded.

Generally, preferred is a method of the present invention, wherein instep (c) the chromium coating has a thickness in a range from 1.1 μm to500 μm, preferably from 2 μm to 450 μm, more preferably from 4 μm to 400μm, even more preferably from 6 μm to 350 μm, yet even more preferablyfrom 8 μm to 300 μm, and most preferably from 10 μm to 250 μm.

In some cases, preferred is a method of the present invention, whereinin step (c) the chromium coating has a thickness of 0.5 μm or more,preferably of 0.75 μm or more, more preferably of 0.9 μm or more, evenmore preferably of 1.0 μm or more, yet even more preferably of 1.5 μm ormore, and most preferably of 2.0 μm or more. In some further cases amethod of the present invention is preferred, wherein in step (c) thechromium coating has a thickness of 15 μm or more, preferably of 20 μmor more.

Preferred is a method of the present invention, wherein in step (c) theconcentration of the at least one additive selected from the groupconsisting of betaines, polymeric glycols, monomeric diols, and mixturesthereof (preferably at least of the betaines), is continually orsemi-continually monitored, wherein

-   -   the monitored concentration is compared to a target        concentration of said at least one additive (preferably of said        betaines), and    -   if the monitored concentration is below the target concentration        than the at least one additive (preferably the betaines) is        added to the electroplating composition.

In particular the aforementioned regarding the betaines in theelectroplating composition applies likewise to the method of the presentinvention.

The present invention according to the third aspect provides a substratewith a surface, wherein the surface of the substrate comprises achromium coating obtained by a method for depositing according to thesecond aspect.

Preferred is a substrate of the present invention, wherein thechromium-coated substrate comprises or is a metal rod.

The aforementioned regarding the electroplating composition of thepresent invention (preferably an electroplating composition as describedas being preferred) and the aforementioned regarding the method of thepresent invention (preferably a method for depositing as described asbeing preferred) preferably applies likewise to the substrate of thepresent invention.

In particular, preferred embodiments of the electroplating compositionof the first aspect and preferred embodiments of the method fordepositing according to the second aspect are also preferred embodimentsfor the substrate according to the third aspect. This applies inparticular and most preferably to the characteristics of the chromiumcoating.

The present invention according to the fourth aspect provides a use ofat least one additive selected from the group consisting of betaines,polymeric glycols, monomeric diols, and mixtures thereof, for increasingthe cathodic current efficiency in an electroplating composition fordepositing a chromium coating on a substrate.

The aforementioned regarding the electroplating composition of thepresent invention (preferably an electroplating composition as describedas being preferred) and the aforementioned regarding the method of thepresent invention (preferably a method for depositing as described asbeing preferred) preferably applies likewise to the use of the presentinvention.

The present invention is described in more detail by the followingnon-limiting examples.

EXAMPLES

For a number of experiments, respective test electroplating compositionswere prepared (volume: appr. 850 mL) comprising 10 g/L to 30 g/Ltrivalent chromium ions (source: basic chromium sulfate), 50 g/L to 250g/L sulfate ions, at least one organic complexing compound (an aliphaticmono carboxylic organic acid), ammonium ions, and bromide ions. Thecompositions did not contain boric acid nor any boron-containingcompounds and no organic compounds with divalent sulfur. The pH was in arange from 5.4 to 5.7.

In a reference electroplating composition (C1) no additive wascontained, defining the reference cathodic current efficiency (CCE). Ina number of further experiments, various additives in a number ofconcentrations were tested (see Table 1 below).

In each test, the respective electroplating composition was subjected toelectroplating in order to obtain a chromium coating on a substrate(mild steel rod with 10 mm diameter). As anodes a graphite anode wasused. Electrodeposition was carried out at 40 A/dm² for 45 minutes at50° C. under mild stirring.

CCE was determined based on the Faraday law and gravimetric analysis.

TABLE 1 Cathodic current efficiency determined on the basis of variousadditives Exp. Additive c [g/L] CCE [%] C1 none — 10.7 1 1,3-propanediol0.2 11.2 2 1,3-propanediol 2 11.2 3 1,3-propanediol 20 13.1 4 PEG 2000.2 13.5 5 PEG 200 2 12.8 6 PEG 200 20 13.9 7 PEG 600 0.2 12.8 8 PEG 6002 11.6 9 PEG 600 20 12.4 10 PEG 1500 0.2 12.8 11 PEG 1500 2 13.1 12 PEG1500 20 13.9 13 N-Dodecyl-N,N-dimethyl-3-ammonio-1- 0.001 12.8propanesulfonate 14 N-Dodecyl-N,N-dimethyl-3-ammonio-1- 0.01 13.1propanesulfonate 15 N-Dodecyl-N,N-dimethyl-3-ammonio-1- 0.1 13.1propanesulfonate 16 N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2- 0.01 11.7hydroxy-3-sulfopropyl) ammonium betaine 17N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2- 0.05 12.5hydroxy-3-sulfopropyl) ammonium betaine 18 N,N-Dimethyl-N-dodecylglycinebetaine 0.05 15.0

Very similar results were obtained with 1,2-propanediol,diethyleneglycol and triethyleneglycol (data not shown), confirming theresults shown in Table 1.

The results in Table 1 show that the tested compound increases the CCEup to 40% compared to the CCE of the reference experiment. Furthermore,according to experiments 1 to 12 a comparatively high additiveconcentration is required compared to the additive utilized inexperiments 13 to 18. Thus, betaines allow to significantly increase theCCE at comparatively low concentrations. It was furthermore observedthat experiments 13 to 18 showed foaming. In further experiments foamingwas limited by adding an anti-foam compound (a tetramethyldecanediolcomprising two iso-propyl moieties (a monomeric C₁₁-C₂₅ diol),solubilized in propylene glycol (a monomeric C₃ diol) prior to addition(total amount of diols in the electroplating composition below 4.0 g/L).

In further comparative examples and in view of U.S. Pat. No. 3,432,408,sulfobetaines as utilized in experiments 13 and 16 were tested in ahexavalent chromium electroplating composition (55° C., 50 A/dm²). TheCCE was appr. 25% without additive and appr. 25% with 0.04 g/L additive.Thus, no significant increase in the CCE was observed, confirming thatthe positive effect of CCE increase is limited to electroplatingcompositions comprising trivalent chromium ions.

1. An electroplating composition for depositing a chromium coating on asubstrate, the composition comprising: (i) trivalent chromium ions, (ii)at least one complexing agent for the trivalent chromium ions, and (iii)at least one additive selected from the group consisting of betaines,polymeric glycols, monomeric diols, and mixtures thereof.
 2. Theelectroplating composition according to claim 1, wherein theelectroplating composition comprises at least one or more than onebetaine independently comprising at least 10 carbon atoms.
 3. Theelectroplating composition according to claim 1, wherein the betainesindependently comprise: a positively charged quaternary nitrogen atom,and a negatively charged sulfonate group or negatively chargedcarboxylate group, with the proviso that the positive charge cannot beremoved by deprotonation.
 4. The electroplating composition according toclaim 1, wherein the betaines are selected from the group consisting ofN-substituted-N,N-Dialkyl-ammonium sulfobetaines.
 5. The electroplatingcomposition according to claim 1, wherein the betaines comprise one ormore of N,N-Dimethyl-N-(3-cocoamidopropyl)-N-(2-hydroxy-3-sulfopropyl)ammonium betaine, N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Decyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, N-Hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate,N-Octadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, andN,N-Dimethyl-N-dodecylglycine betaine.
 6. The electroplating compositionaccording to claim 1, wherein the electroplating composition comprisesat least one betaine and in addition one or more than one monomericdiol.
 7. The electroplating composition according to claim 1, whereinthe electroplating composition comprises the betaines in a totalconcentration in a range from 0.0005 g/L to 1 g/L, based on the totalvolume of the electroplating composition.
 8. The electroplatingcomposition according to claim 1, wherein the electroplating compositionhas a pH in a range from 4.1 to 7.0.
 9. The electroplating compositionaccording to claim 1, wherein the electroplating composition comprisesone or more than one betaine and one or more than one monomeric diol,with the proviso that the one or more than one monomeric diol comprisesone or more than one C₁₁-C₂₅ diol comprising one, two or more than twoiso-propyl moieties.
 10. A method for depositing a chromium coating on asubstrate, the method comprising the following steps: (a) providing thesubstrate, (b) providing an electroplating composition for depositing achromium coating on the substrate, the composition comprising: (i)trivalent chromium ions, (ii) at least one complexing agent for thetrivalent chromium ions, and (iii) at least one additive selected fromthe group consisting of betaines, polymeric glycols, monomeric diols,and mixtures thereof, (c) contacting the substrate with saidelectroplating composition and applying an electrical current such thatthe chromium coating is deposited on at least one surface of saidsubstrate.
 11. The method of claim 10, wherein in step (c) the cathodiccurrent efficiency (CCE) is 11% or more.
 12. The method of claim 10,wherein in step (c) the chromium coating has a thickness in a range from1.1 μm to 500 μm.
 13. The method of claim 10, wherein in step (c) theconcentration of the at least one additive selected from the groupconsisting of betaines, polymeric glycols, monomeric diols, and mixturesthereof, is continually or semi-continually monitored, wherein themonitored concentration is compared to a target concentration of said atleast one additive, and if the monitored concentration is below thetarget concentration than the at least one additive is added to theelectroplating composition.
 14. A substrate with a surface, wherein thesurface of the substrate comprises a chromium coating obtained by amethod for depositing according to claim
 10. 15. (canceled)
 16. Theelectroplating composition according to claim 1, wherein theelectroplating composition comprises the betaines in a totalconcentration in a range from 0.01 g/L to 0.2 g/L, based on the totalvolume of the electroplating composition.
 17. The method of claim 10,wherein in step (c) the chromium coating has a thickness in a range from10 μm to 250 μm.